Carrierless amplitude modulation/phase modulation (CAP) is a bandwidth-efficient two-dimensional passband line code. (Additional information on a CAP communications system can be found in J. J. Werner, "Tutorial on Carrierless AM/PM-Part I-Fundamentals and Digital CAP Transmitter," Contribution to ANSI X3T9.5 TP/PMD Working Group, Minneapolis, Jun. 23, 1992.) CAP is closely related to the more familiar quadrature amplitude modulation (QAM) transmission scheme. In voiceband modems, QAM has been used for over 25 years, while CAP has been used for over 15 years. However, CAP is simpler to implement digitally. Illustrative prior art transceiver structures for the QAM and CAP transmission schemes are shown in FIGS. 1 and 2, respectively. Both FIGS. 1 and 2 illustrate two-dimensional encoding where a complex symbol, A.sub.n, is applied to the transmitter portion (where A.sub.n =a.sub.n +jb.sub.n), and a recovered complex symbol, A.sub.n, is provided by the receiver portion, where A.sub.n =a.sub.n +jb.sub.n. With respect to other notation used in these FIGS., g(t) (e.g., see FIG. 1) is an impulse response of a baseband shaping filter, e.sub.in (t) and e.sub.qu (t) are equalizers for the in-phase and quadrature components, respectively, and p(t) and p(t) are impulse responses of a shaping filter which form a Hilbert pair (e.g., see FIG. 2).
As can be observed from FIG. 1, the conventional QAM transceiver structure requires a modulator 1 and a demodulator 2 at the transmitter and receiver, respectively. In contrast, the CAP transceiver of FIG. 2 does not require a modulator and a demodulator. Generally speaking, the CAP system of FIG. 2 is not compatible with the QAM system of FIG. 1, but provides the same theoretical performance as QAM and is simpler to implement digitally than QAM. Indeed, the CAP structure of FIG. 2 can be modified into a simpler QAM-only transceiver, which is shown in FIG. 3.
Currently, some broadband access applications, such as VDSL (Very high rate Digital Subscriber Line), may require either a CAP receiver or a QAM receiver. Some in the art have proposed simply putting both the CAP receiver and the QAM receiver into one receiver--in effect having a dual structure receiver with a CAP section (having its own equalizer) and a separate QAM section (with its own equalizer). To further complicate matters, this dual structure receiver may require the use of blind equalization techniques in both the QAM section and the CAP section. In this case, there is no training signal for the dual structure receiver to use to identify the type of modulation. As such, the dual structure receiver must first independently converge both the equalizer in the QAM section and the equalizer in the CAP section, and then make a decision as to the type of modulation--all of which may cause significant timing overhead.